I don't understand what they want
Most people don't understand what other people want. To start, don't try to figure out what they want. Focus on what you want. Do they meet your wants/needs in a potential partner. If they do ask them what they're looking for in a potential partner.
As is commonly discussed, communication is the most important part of a functioning relationship. If you don't know what you want, how can you communicate it to your partner.
If you don't ask and they don't tell you what they want, there is no tried and true way to find out.
Dating is about finding someone compatible, if we could tell you what all men and all women want wouldn't every man and every woman automatically be compatible?
Find your person by meaningfully talking to people, not asking the internet to give you catch-all tips and tricks.